The manufacture of semi-conductor devices such as diodes and transistors typically requires the deposition of dielectric materials such as polycrystalline silicon, silicon nitride and silicon dioxide on the surfaces of thin silicon wafers. The thin layer deposition of these materials involves rapid heating and cooling cycles in an electrically heated furnace (or "diffusion process tube") at temperatures typically ranging from 250 to 1000 C. When dielectric precursor gases are fed into a diffusion process tube heated to these temperatures, the gases react and deposit the dielectric reaction product on the surface of the silicon wafer.
During the deposition step, the silicon wafers are supported in vertical or horizontal kiln furniture (or "wafer boats") placed within the process tube. The wafer boat and process tube are typically made of a material which has excellent thermal shock resistance, high mechanical strength, an ability to retain its shape through a large number of heating and cooling cycles, and which does not out-gas (i.e., introduce any undesirable impurities into the atmosphere of the kiln during firing operations). One material which meets these requirements is silicon carbide.
When the diffusion component is used in high temperature applications, metallic impurities contained therein often diffuse through the body and contaminate the silicon wafer. Accordingly, as a general rule, it is desirable to use as pure a diffusion component as possible, and so raw silicon carbide powders are generally purified to reduce the contaminant levels therein. However, in many relatively low temperature applications wherein the danger of metallic diffusion and contamination is not so great, a less pure diffusion component can be used. In one method of making these diffusion components, a slurry comprising a bimodal blend of untreated silicon carbide powders is slip cast to form a green body, and the green body is fired above about 1900 C. to promote recrystallization. Although use of low purity powders in this process dramatically lowers the cost of making silicon carbide diffusion components for these applications, it has been found that many components so made crack during recrystallization. Since these cracks render the component essentially useless, the cost of supplying less pure diffusion components increases.